This invention relates to a fused salt thermal device which is capable of generating electricity.
As is well known in the art of thermal devices, an inorganic fusible salt electrolyte disposed between two electrodes will not be capable of generating any electricity if the temperature of the salt electrolyte is below its activation temperature. Upon reception of sufficient thermal energy to raise the temperature of the salt electrolyte to its activation temperature where the salt becomes molten or fused, the fused salt electrolyte becomes a good ionic conductor permitting the thermal cell to generate electricity in a closed system without addition of further electrolyte. Such thermal devices remain active and generate electricity only while the electrolyte is at or above its activation temperature. The inorganic fusible salt electrolyte has an activation temperature which is usually, but not necessarily, the temperature at which the salt becomes a liquid or fuses. As long as the electrolyte remains at or above its activation temperature, the thermal device will generate electricity.
Since the activation temperature of the electrolyte is generally the melting point of the fusible salt electrolyte, a considerable degree of latitude is permitted in selecting a cell activation point by choosing a fusible salt electrolyte which has a melting point above the ambient temperature of the surroundings udring the inactive state. The fused salt electrolyte is also selected in accordance with the temperature and the intensity of the thermal energy available to activate the cell. Thus, depending on the materials selected for the electrodes, the cell design, and the intended use of the cell, an inorganic fusible salt electrolyte may be selected to achieve a desired activation temperature.
With reference to sources of electric current, the word "cell" usually applies to a single element comprising a pair of electrodes and an electrolyte that interact to produce electricity. Generally, the term "battery" contemplates an assembly of two or more such cells. In voltaic cells the positive electrode is termed the cathode while the negative electrode is termed the anode; conversely, in electrolysis cells and secondary cells or batteries, the terminology is reversed. For the sake of consistency throughout this specification the terminology of a voltaic cell will be used, where the negative electrode is the anode and the positive electrode is the cathode.
Generally, the inorganic fusible salt electrolyte employed in thermal devices is hygroscopic or delinquescent such that fabrication of the thermal device requires an atmosphere where the humidity and moisture are controlled. Also, a cell employing such an electrolyte must seal the electrolyte from contact with the ambient surroundings so that the electrolyte does not absorb moisture which deteriorates the effect of the fused salt electrolyte.
Another problem encountered in the operation of fused salt thermal devices is the conflict between attempting to locate the positive and negative electrodes as close together as possible in order to maximize current output while also maximizing the amount of electrolyte in order to accommodate deterioration and degeneration of the electrolyte and to provide maximum output life without additional thermal energy from an external source. These conflicting objectives are further compounded in the construction of a thermal cell or battery where the electrolyte for each cell has heretofore been contained totally between the two electrodes. See, for example, U.S. Pat. Nos. 3,719,527 and 3,575,714. Heat fusible salt thermal reservoirs are known in thermal cells, but the salt of the reservoir is a separate entity from the electrolyte. U.S. Pat. Nos. 3,899,353 and 3,677,822. In electrochemical fuel cells, electrolyte reservoirs are known for providing additional electrolyte for circulation through the electrodes into a narrow active cell space with a decreased internal resistance (U.S. Pat. No. 3,769,090), but the circulating electrolyte is employed to remove heat rather than contain it as desired in thermal cells and to accommodate the generation of gases which cause the problem of bubble pressure not encountered in thermal cells.
Another problem encountered in fused salt thermal devices is that suitable inorganic fusible salt electrolytes are generally very strong corrosive and oxidizing agents, and therefore, care must be taken in constructing the cell and handling the fused electrolyte. The oxidizing effect of inorganic fusible salt electrolytes can also adversely affect the electrical output of the thermal device.
With regard to materials, fused salt thermal cells having magnesium and carbon electrodes are standard and well known in the thermal cell field as shown by U.S. Pat. No. 2,291,739 and by Goodrich, Robert B., et al, "Thermal Batteries," Journal of the Electrochemical Society, Volume 99, No. 8, page 207C, August, 1952. It is known that the carbon electrode may be porous carbon or a carbon cloth, as exemplified by U.S. Pat. No. 3,573,986. Although the usual electrolyte in thermal cells is some type of halide salt such as sodium chloride, various nitrate electrolytes are known, particularly for their advantage in providing a low melting point electrolyte. "High-Energy Batteries," Jasinski, Raymond, Plenum Press, New York, 1967. Specific nitrates that are known and disclosed in the art are lithium nitrate, sodium nitrate, potasium nitrate, and silver nitrate. U.S. Pat. Nos. 1,406,352, 3,575,714, and 3,719,527. However these electrolytes are more corrosive, are stronger oxidizing agents, and have higher melting points than chromic nitrate which has heretofore, insofar as the present inventor is aware, not been known or used in the thermal cell field. Furthermore, chromic nitrate as used in combination with carbon, preferably carbon cloth, and magnesium electrodes is not known insofar as the present inventor is aware and has additional advantages that will be described in more detail below.